Optical quality diamond material

ABSTRACT

A CVD single crystal diamond material suitable for use in, or as, an optical device or element. It is suitable for use in a wide range of optical applications such as, for example, optical windows, laser windows, optical reflectors, optical refractors and gratings, and etalons. The CVD diamond material is produced by a CVD method in the presence of a controlled low level of nitrogen to control the development of crystal defects and thus achieve a diamond material having key characteristics for optical applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/462,690, filed on Aug. 19, 2014, which is a division of U.S. patentapplication Ser. No. 12/690,991, filed on Jan. 21, 2010, issued as U.S.Pat. No. 8,936,774 on Jan. 20, 2015, which is a division of U.S. patentapplication Ser. No. 10/717,566, filed on Nov. 21, 2003, issued as U.S.Pat. No. 7,740,824 on Jun. 22, 2010, which claims the benefit of UnitedKingdom Patent Application No. 0227261.5, filed on Nov. 21, 2002, eachof which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to chemical vapour deposition (CVD) diamondmaterial, its production and optical devices and elements arising fromthis material.

There is a range of optical devices which, as a result of their uniquerequirements, place high demands on the material used for them. Examplesinclude laser windows, where high intensity beams need to passundisturbed through a window which is required to provide some form ofisolation, and other devices such as optical reflectors, diffractiongratings and etalons.

Depending on the particular application, key properties that may play arole in the selection or manufacturing of an appropriate materialinclude low and uniform birefringence, uniform and high refractiveindex, low induced birefringence or refractive index variation as afunction of strain, low and uniform optical absorption, low and uniformoptical scatter, high optical (laser) damage threshold, high thermalconductivity (minimising temperature variation within the opticalelement), an ability to be processed to show a high surface polishtogether with high parallelism and flatness, mechanical strength,abrasion resistance, chemical inertness, and repeatability in thematerial parameters so that it is reliable in the application.

Many materials fulfill one or more of these requirements, but mostapplications require more than one, and often the material chosen is acompromise, limiting the final performance.

SUMMARY OF THE INVENTION

According to the present invention, a CVD single crystal diamondmaterial shows at least one, preferably at least two, more preferably atleast three, and even more preferably at least four of the followingcharacteristics, when measured at room temperature (nominally 20° C.):

-   -   1) High optical homogeneity, with the transmitted wavefront (as        for example measured by a ZYGO GPI phase shifting 633 nm        Fizeau-type laser interferometer) differing from the expected        geometrical wavefront during transmission through diamond of a        specified thickness of at least 0.5 mm, preferably at least 0.8        mm and more preferably at least 1.2 mm, processed to an        appropriate flatness, and measured over a specified area of at        least 1.3 mm×1.3 mm, and preferably at least 2.5 mm×2.5 mm, and        more preferably at least 4.0 mm×4.0 mm, by less than 2 fringes,        and preferably by less than 1 fringe, and more preferably by        less than 0.5 fringes, and even more preferably by less than 0.2        fringes, where 1 fringe corresponds to a difference in optical        path length equal to ½ of the measurement wavelength of 633 nm.    -   2) An effective refractive index in samples of a specified        thickness of at least 0.5 mm, preferably at least 0.8 mm and        more preferably at least 1.2 mm, and measured over a specified        area of at least 1.3 mm×1.3 mm, and preferably at least 2.5        mm×2.5 mm, and more preferably at least 4 mm×4 mm, which has a        value of 2.3964 to within an accuracy of +/−0.002, and        preferably to within an accuracy of +/−0.001, and more        preferably to within an accuracy of +/−0.0005, when measured at        wavelengths near 1.55 μm by scanning the frequency of the laser        beam incident on the sample in the form of an etalon over the        frequency range of 197-192 THz, recording the transmission of        the sample etalon as a function of frequency, and applying the        formula for the Free Spectral Range (equation 1 defined later).        Those skilled in the art will understand that the value of        2.3964 is based on the diamond consisting of carbon isotopes in        their natural abundance ratio, and that the value of 2.3964 will        vary as the isotopic composition of the diamond varies.    -   3) Low optical birefringence, indicative of low strain such that        in samples of a specified thickness of at least 0.5 mm,        preferably at least 0.8 mm and more preferably at least 1.2 mm,        measured over a specified area of at least 1.3 mm×1.3 mm, and        preferably at least 2.5 mm×2.5 mm, and more preferably at least        4 mm×4 mm, the modulus of the sine of the phase shift, |sin δ|,        as measured by a Deltascan or similar instrument with similar        resolution using radiation in the range 545-615 nm and        preferably 589.6 nm does not exceed certain limits.        Specifically, these limits are that the modulus of the sine of        the phase shift, |sin δ| for at least 98%, and more preferably        for at least 99% and even more preferably for 100% of the        analysed area of the sample remains in first order (δ does not        exceed π/2) and that |sin δ| does not exceed 0.9, and preferably        does not exceed 0.6, and more preferably does not exceed 0.4,        and more preferably does not exceed 0.3, and more preferably        does not exceed 0.2.    -   4) A combination of optical properties such that a suitably        prepared diamond plate in the form of an etalon of a specified        thickness of at least 0.5 mm, preferably at least 0.8 mm and        more preferably at least 1.2 mm, and measured using a laser beam        with a wavelength near 1.55 μm and a nominal diameter of 1.2 mm,        over a specified area of at least 1.3 mm×1.3 mm, and preferably        2.5 mm×2.5 mm, and more preferably at least 4 mm×4 mm, exhibits        a free spectral range (FSR) which, when measured at different        positions over the plate varies by less than 5×10⁻³ cm⁻¹, and        preferably by less than 2×10⁻³ cm⁻¹, and more preferably by less        than 5×10⁻⁴ cm⁻¹, and even more preferably by less than 2×10⁻⁴        cm⁻¹.    -   5) A combination of optical properties such that a suitably        prepared diamond plate in the form of a Fabry-Perot solid etalon        of a specified thickness of at least 0.5 mm, preferably at least        0.8 mm and more preferably at least 1.2 mm, and measured using a        laser beam with a wavelength near 1.55 μm and a nominal diameter        of 1.2 mm, over a specified area of at least 1.3 mm×1.3 mm and        preferably at least 2.5 mm×2.5 mm, and more preferably at least        4 mm×4 mm, and which has no coatings applied to the optically        prepared surfaces, exhibits when measured at different positions        over the plate a contrast ratio exceeding 1.5 and preferably        exceeding 1.6 and more preferably exceeding 1.7 and even more        preferably exceeding 1.8 and most preferably exceeding 1.9. The        contrast ratio is defined as the ratio of the value of the        etalon transmission at an incident laser wavelength near 1.55 μm        where the transmission has a maximum value to the value of the        etalon transmission at an incident laser wavelength near 1.55 μm        where the transmission has a minimum value and the transmission        value is defined as the ratio of the optical power of a laser        beam that is transmitted through the etalon to the laser power        that is incident on the etalon.    -   6) A combination of optical properties such that a suitably        prepared diamond plate in the form of an etalon of a specified        thickness of at least 0.5 mm, preferably at least 0.8 mm and        more preferably at least 1.2 mm, and measured using a laser beam        with a wavelength near 1.55 μm and a diameter of 1.2 mm, over a        specified are of at least 1.3 mm×1.3 mm, and preferably at least        2.5 mm×2.5 mm, and more preferably at least 4 mm×4 mm, exhibits        an insertion loss not exceeding 3 dB and preferably not        exceeding 1 dB and more preferably not exceeding 0.5 dB and even        more preferably not exceeding 0.3 dB.    -   7) Low and uniform optical absorption, such that a sample of a        specified thickness of at least 0.5 mm, preferably at least 0.8        mm and more preferably at least 1.2 mm, has an optical        absorption coefficient at a wavelength of 10.6 μm measured near        20° C. of less than 0.04 cm⁻¹, and preferably less than 0.03        cm⁻¹, and more preferably less than 0.027 cm⁻¹, and even more        preferably less than 0.025 cm⁻¹.    -   8) Low and uniform optical absorption such that a sample of a        specified thickness of at least 0.5 mm, preferably at least 0.8        mm and more preferably at least 1.2 mm, has an optical        absorption coefficient at 1.06 μm of less than 0.09 cm⁻¹, and        preferably less than 0.05 cm⁻¹, and more preferably less than        0.02 cm⁻¹, and even more preferably less than 0.01 cm⁻¹.    -   9) Low and uniform optical scatter, such that for a sample of a        specified thickness of at least 0.5 mm, preferably at least 0.8        mm and more preferably at least 1.2 mm, and measured over a        specified area of at least 1.3 mm×1.3 mm, and preferably at        least 2.5 mm×2.5 mm, and more preferably at least 4 mm×4 mm, the        forward scatter at a wavelength of 0.63 μm, integrated over a        solid angle from 0.3° to 45° from the transmitted beam, is less        than 0.2%, and preferably less than 0.1%, and more preferably        less than 0.05%, and even more preferably less than 0.03%.    -   10) Low and uniform optical scatter, such that for a sample of a        specified thickness of at least 0.5 mm, preferably at least 0.8        mm and more preferably at least 1.2 mm, and measured over a        specified area of at least 1.3 mm×1.3 mm, and preferably at        least 2.5 mm×2.5 mm, and more preferably at least 4 mm×4 mm, the        forward scatter at a wavelength of 10.6 μm, integrated over the        solid angle from 1.1° to 45° from the transmitted beam, is less        than 0.004%, and preferably less than 0.002%, and more        preferably less than 0.001%, and even more preferably less than        0.0005%.    -   11) A high laser damage threshold, such that at a wavelength of        10.6 μm using a Gaussian beam profile with a primary pulse spike        of 50-100 ns and normalised to a 100 μm 1/e spot size, the mean        of the lowest incident peak energy density that causes damage        and the highest incident peak energy density that does not cause        damage is greater than 120 Jcm⁻², and preferably greater than        220 Jcm⁻², and more preferably greater than 320 Jcm⁻², and even        more preferably greater than 420 Jcm⁻².    -   12) A high laser damage threshold, such that at a wavelength of        1.06 μm using a Gaussian beam profile, with a primary pulse        spike of 10-50 ns and more preferably 20-40 ns, and normalised        to a 100 μm 1/e spot size, the mean of the lowest incident peak        energy density that causes damage and the highest incident peak        energy density that does not cause damage is greater than 35        Jcm⁻², and preferably greater than 50 Jcm⁻², and more preferably        greater than 80 Jcm⁻², and even more preferably greater than 120        Jcm⁻², and even more preferably greater than 150 Jcm⁻².    -   13) High thermal conductivity, with a value for material        composed of carbon in its natural isotopic abundance which when        measured at 20° C. is greater than 1500 Wm⁻¹ preferably greater        than 1800 Wm⁻¹K⁻¹, more preferably greater than 2100 Wm⁻¹K⁻¹,        even more preferably greater than 2300 Wm⁻¹K⁻¹, and even more        preferably greater than 2500 Wm⁻¹K. Those skilled in the art        will understand that this is based on the diamond containing        carbon isotopes in their natural abundance ratio, and that the        figures will vary as the isotopic composition of the diamond        varies.    -   14) An ability to be processed to show a high surface polish        over a specified area of at least 1.3 mm×1.3 mm, and preferably        at least 2.5 mm×2.5 mm, and more preferably at least 4.0 mm×4.0        mm, with an R_(a) (arithmetic mean of the absolute deviation        from the mean line through the profile) less than 2 nm, and        preferably less than 1 nm, and more preferably less than 0.6 nm.    -   15) An ability to be processed to show a high parallelism, with        a parallelism over a specified area of at least 1.3 mm×1.3 mm,        and preferably at least 2.5 mm×2.5 mm, and more preferably at        least 4.0 mm×4.0 mm, which is better than 1 arc minute, and        preferably better than ±30 arc seconds, and more preferably        better than ±15 arc seconds, and even more preferably better        than ±5 arc seconds.    -   16) An ability to be processed to show a high flatness, with a        flatness measured using 633 nm radiation over a specified area        of at least 1.3 mm×1.3 mm, and preferably at least 2.5 mm×2.5        mm, and more preferably at least 4.0 mm×4.0 mm, which is better        than 10 fringes, and preferably better than 1 fringe, and more        preferably better than 0.3 fringes.    -   17) A mechanical design strength, obtained from measurements        made using a single cantilever beam technique with individual        sample dimensions of 5.0 mm by 3.0 mm by 0.17-0.35 mm (length by        breadth by thickness), in which at least 70% and preferably at        least 80%, and more preferably at least 90% of samples tested        over a batch size of at least 8, and preferably at least 10, and        more preferably at least 15, will only fail at strength values        of at least of 2.5 GPa, and preferably of at least of 3.0 GPa,        and more preferably of at least 3.5 Gpa.    -   18) A variation in refractive index over a volume of interest,        said volume comprising a layer of a specified thickness of at        least 0.5 mm, preferably at least 0.8 mm and more preferably at        least 1.2 mm, and characterised by fabricating into one or more        plates, which has a standard deviation in the refractive index        which is less than 0.002, and preferably less than 0.001, and        more preferably less than 0.0005, when measured at wavelengths        near 1.55 μm over an area of at least 1.3 mm×1.3 mm, and        preferably at least 2.5 mm×2.5 mm, and more preferably at least        4 mm×4 mm, by scanning the frequency of the laser beam incident        on the sample in the form of an etalon over a frequency range of        197-192 THz, recording the transmission of the sample etalon as        a function of frequency, and applying the formula for the Free        Spectral Range (equation 1 defined later).    -   19) An effective refractive index in samples of a thickness of        at least 0.5 mm, preferably at least 0.8 mm and more preferably        at least 1.2 mm, and measured over a specified area of at least        1.3 mm×1.3 mm, and preferably at least 2.5 mm×2.5 mm, and more        preferably at least 4 mm×4 mm, which has a value of 2.39695 to        within an accuracy of +/−0.001, and more preferably to within an        accuracy of +/−0.0005, when measured at wavelengths near 1.55 μm        by scanning the frequency of the laser beam incident on the        sample in the form of an etalon over the frequency range of        197-192 THz, recording the transmission of the sample etalon as        a function of frequency, and applying the formula for the Free        Spectral Range (equation 1 defined later). Those skilled in the        art will understand that the value of 2.39695 is based on the        diamond consisting of carbon isotopes in their natural abundance        ratio, and that the value of 2.39695 will vary as the isotopic        composition of the diamond varies.    -   20) Low and uniform optical scatter, such that for a sample of a        specified thickness of at least 0.4 mm, preferably at least 0.8        mm and more preferably at least 1.2 mm thick, and measured over        a specified area of at least 1.3 mm×1.3 mm, and preferably at        least 2.5 mm×2.5 mm, and more preferably at least 4 mm×4 mm, the        forward scatter at 1.064 μm measured by the method described        herein, integrated over a solid angle from 3.5° to 87.5° from        the transmitted beam, is less than 0.4%, and preferably less        than 0.2%, and more preferably less than 0.1%, and even more        preferably less than 0.05%.    -   21) Low luminescence under optical excitation at 514 nm, such        that the Raman normalised intensity of either, and more        preferably both, of the 575 nm photoluminescence (PL) line and        the 637 nm PL line, is less than 40, and preferably less than 10        and more preferably less than 3 and more preferably less than 1.    -   22) Low optical birefringence, indicative of low strain such        that in samples of a specified thickness of at least 0.5 mm,        preferably at least 0.8 mm and more preferably at least 1.2 mm,        measured over a specified area of at least 1.3 mm×1.3 mm, and        preferably at least 2.5 mm×2.5 mm, and more preferably at least        4 mm×4 mm, the maximum value of Δn_([average]), the average        value of the difference between the refractive index for light        polarised parallel to the slow and fast axes averaged over the        sample thickness, as measured by a Deltascan or similar        instrument with similar resolution using radiation in the range        545-615 nm and preferably 589.6 nm does not exceed certain        limits. Specifically, these limits are that for at least 98%,        and more preferably for at least 99% and even more preferably        for 100% of the analysed area of the sample, the birefringence        remains in first order (δ does not exceed π/2) and that        Δn_([average]) does not exceed 1.5×10⁻⁴, and preferably does not        exceed 5×10⁻⁵, and more preferably does not exceed 2×10⁻⁵, and        more preferably does not exceed 1×10⁻⁵.

The diamond material is preferably formed into a mechanical layer or anoptical layer or polished gemstone, and more preferably an opticallayer, and preferably exceeds one, more preferably two, and even morepreferably three, of the following dimensions:

-   -   a) a lateral dimension of 1 mm, preferably 2 mm, more preferably        5 mm and even more preferably 8 mm,    -   b) a second orthogonal lateral dimension of 1 mm, preferably 2        mm, more preferably 5 mm and even more preferably 8 mm,    -   c) a thickness of 0.1 mm, preferably 0.3 mm, more preferably 0.5        mm, and even more preferably 0.8 mm.

The invention extends to a single crystal CVD diamond material asdescribed above for use in, or as, an optical device or element. Suchdevice or element may be suitable for use in a wide range of opticalapplications, including, but not limited to, optical windows, laserwindows, optical reflectors, optical refractors and gratings, andetalons. For applications requiring reflection at one or more surfacessuch as beam splitters or etalons, the diamond may be used uncoated onthese surfaces. In addition the material is advantageous as a polishedgemstone, in which form it may be initially produced as a much thickerlayer prior to polishing, typically 2.5 mm and more typically 3.5 mmthick or greater. Properties particularly applicable to this applicationinclude the uniformity of optical characteristics, the low scatter andabsorption, and the ease of processing and the quality of the processedsurface, which, particularly in combination, provides for a morebrilliant stone.

The diamond material of the invention can be tailored to specificapplications, and although it may not be endowed with all of the aboveproperties in all cases, in many applications it is the ability of thediamond material to show a substantial set or particular combination ofthe above properties which makes its use particularly beneficial. Forexample, for use as an etalon, the material may require opticalhomogeneity, low absorption, high thermal conductivity, and the abilityto be processed flat and parallel, but laser damage thresholds andmechanical strength may be less important. Conversely, in application asa viewing or optical access window, the strength may be important, asmay be the scatter, the absorption, and characteristics affecting imagequality.

An optical device which includes or comprises a CVD diamond material ofthe invention may have attached to it or built into it either a heatsource or a temperature or other measuring device, or both. The heatsource provides the ability to alter the temperature of the opticaldevice, and thus any temperature dependent properties, and thetemperature sensor a means by which to monitor this and in someinstances provide feedback control. This technique is particularlyapplicable to diamond because its high thermal conductivity ensures theinput heat is distributed uniformly very rapidly. Particular embodimentsof this form of the invention may be the incorporation of doped layersor tracks using dopants such as boron to form the heater elements, andalso further doped structures for the measurement of the temperature.Such doped structures could be produced by ion implantation or by othersurface processing techniques.

The material may exhibit the beneficial properties after growth, aftersuitable shape and surface processing as required, or it may beprocessed by annealing to further enhance specific properties.

In application, the material may be further treated, such treatmentsincluding mountings, metallisations (such as for gratings), coatings(such as anti-reflection coatings), etching the surface to a specifictopography (such as for diffractive optics), or the like.

According to another aspect of the invention, a method of producing aCVD diamond material suitable for optical applications comprises growinga single crystal diamond on a substrate with low crystal defect densityby a CVD method in the presence of a controlled low level of nitrogen tocontrol the development of crystal defects.

The level of nitrogen used in the method is selected to be sufficient toprevent or reduce local strain generating defects whilst being lowenough to prevent or reduce deleterious absorptions and crystal qualitydegradation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 is a schematic side view of a solid etalon showing a typicalreflectance and transmittance pattern of a beam incident on a firstsurface thereof;

FIG. 2 is a graphical representation of a typical transmissioncharacteristic of a solid etalon;

FIG. 3 is a graphical representation of a typical reflectivitycharacteristic of a Gires-Tournois etalon;

FIG. 4 is a diagrammatic representation of a system for measuringoptical scatter at 1.06 μm in small diamond samples;

FIG. 5 is a diagrammatic representation of the conversion ofone-dimensional data obtained from the system of FIG. 4 intotwo-dimensional data;

FIG. 6 is a graphical representation of the measurement made of FSR as afunction of the inverse of the sample thickness for diamond samples asdescribed in Example 4;

FIG. 7 is a graphical representation of the contrast ratios of testedetalon plates of the invention as described in Example 5;

FIG. 8 is a graphical representation of measured surface flatness valuesof tested etalon plates of the invention as described in Example 7; and

FIG. 9 is a graphical representation of the FSR data measured asdescribed in Example 15. Data are shown for each of 11 CVD singlecrystal plates, showing the standard deviation of the FSR measurementsmade on the plate and the maximum deviation of any measurement from theaverage.

FIG. 10 is a Deltascan map of a sample E4.1, showing the maximum valueof |sin δ|in each frame of 1 mm×0.75 mm.

FIG. 11 is a Deltascan map of a sample E4.2, showing the maximum valueof |sin δ|in each frame of 1 mm×0.75 mm.

FIG. 12 is a Deltascan map of a sample E4.3, showing the maximum valueof |sin δ|in each frame of 1 mm×0.75 mm.

FIG. 13 is a Deltascan map of a sample E10.1, with the viewing directionparallel to a 1.31 mm dimension of the plate, showing the maximum valueof |sin δ|in each frame of 1 mm×0.75 mm.

DESCRIPTION OF EMBODIMENTS

The CVD diamond material of the invention is produced by a CVD method inthe presence of a controlled low level of nitrogen. The level ofnitrogen utilised is critical in controlling the development of crystaldefects and thus achieving a diamond material having the keycharacteristics of the invention. It has been found that material grownwith high levels of nitrogen show deleterious absorptions. High levelsof nitrogen may also degrade the crystal quality of the material.Conversely, material grown under conditions with essentially nonitrogen, or less than 300 ppb of nitrogen has a comparatively higherlevel of local strain generating defects, which affect directly orindirectly many of the high performance optical properties of thediamond. The exact mechanism of this is not well understood, but thefollowing observations have been made. In what follows the word‘dislocations’ is intended to cover both isolated dislocations anddislocation bundles where dislocations are grouped together.

No material can be made entirely free of dislocations and other crystaldefects over large volumes. The sensitivity of different properties tosuch crystal defects varies. For example, the average refractive indexis relatively insensitive, although local variations of this are quitesensitive. Engineering optical properties in CVD diamond, at the highlevel of precision required, appear to be extremely sensitive todislocations.

The method of this invention is primarily applicable to growth on a{100} substrate face, and this is assumed unless otherwise stated,although the general method may be capable of growth of optical gradediamond on other specific substrate orientations.

In the absence of sufficient nitrogen in the gas mixture of the growthprocess, pits form on the {100} growth surface around dislocations whichpre-exist in the substrate material or are generated at thesubstrate/growth interface. Whether because of these pits or otherwise,dislocations can slowly multiply during growth. To a certain extent thisprocess is exponential, with the rate of dislocation multiplicationdependent on the existing local dislocation density and arrangement.

In the presence of small amounts of nitrogen, relative facet growthrates are changed and these pits can be removed. Whether because of theabsence of these pits or otherwise, dislocation multiplication can bereduced or totally avoided.

These pits may also be responsible for the inclusion of other defectsand impurities in the material.

A further effect has been noted, which may form an important part of theprocess. At typical process conditions without nitrogen the epitaxialgrowth takes place with the progression of <110> surface steps movingacross the substrate surface. These steps are typically just visibleunder a standard optical microscope, although their presence isgenerally best confirmed using Nomarski techniques or other highsensitivity techniques. Under the right conditions, increasing thenitrogen within the very low concentration levels of this invention,does not affect the morphology of these surface steps. With these stepspresent, the uptake of N into the diamond is generally low.

As the nitrogen level increases, the surface growth mechanism undergoesa transition. The surface steps of the diamond become more random andmore generally centered around the <100> orientation, at least on amacroscopic scale, and the steps become larger and rougher. These stepsare easily seen by microscope, and can often be seen by eye. In thepresence of these steps nitrogen incorporation increases substantiallyand is generally non-uniform on a microscopic scale. The concentrationof nitrogen at which this transition occurs is a sensitive function ofthe growth conditions, including temperature and pressure, but istypically around 1.8 ppm (of total gas concentration, when using N₂) forthe processes described herein. For small excursions above this limit,some of the benefits of the method of the invention may still berealised, particularly for example the mechanical strength or thesurface processing, but properties such as optical absorption areadversely affected by significant nitrogen incorporation, which easilyoccurs once <100> steps are present.

The lower bound to the concentration of N in the process gas is thus setby the need to limit pitting and control the strain generating defects,and the upper bound to the concentration of nitrogen in the process gasset by the onset of deleterious absorptions and other types of defectgeneration, and the role that the change in surface step growthmechanism has on these. However, these bounds are process dependent,such that they may vary according to the process conditions used,including the actual gaseous source of N, and also the specific materialproperties required, and are best illustrated by way of example.Typically however in the method of the invention, the upper bound on thenitrogen level in the gas phase (ppm N₂, or the equivalent of the actualnitrogen source used to provide the same total N) is 5 ppm, andpreferably 2 ppm, and more preferably 1.5 ppm, and even more preferably1 ppm. The lower bound on the nitrogen level in the gas phase (ppm N₂,or the equivalent of the actual nitrogen source used to provide the sametotal nitrogen) is greater than 300 ppb, and preferably greater than 500ppb, and more preferably greater than 700 ppb, and even more preferablygreater than 800 ppb.

The material can also be characterized in terms of the typical nitrogenconcentration in the solid, although the relationship between thisconcentration and that of the nitrogen in the gas phase is a sensitivefunction of the detailed growth conditions. Typically the singlesubstitutional nitrogen concentration as measured by electronparamagnetic resonance (EPR), exceeds 3×10¹⁵ atoms/cm³, and moretypically exceeds 1×10¹⁶ atoms/cm³, and even more typically exceeds5×10¹⁶ atoms/cm³. Furthermore, this concentration of the singlesubstitutional nitrogen is typically less than 5×10¹⁷ atoms/cm³, andmore typically is less than 2×10¹⁷ atoms/cm³.

Using the above conditions it has been possible to produce the singlecrystal CVD diamond material of the invention, typically in layer form,with advantageous optical properties.

It is important for the production of a diamond optical layer of theinvention that growth of a layer of single crystal CVD diamond takesplace on a diamond surface which is substantially free of crystaldefects. In this context, defects primarily mean dislocations and microcracks, but also include twin boundaries, point defects notintrinsically associated with the dopant N atoms, low angle boundariesand any other extended disruption to the crystal lattice. Preferably thesubstrate is a low birefringence type Ia natural, Ib or IIa highpressure/high temperature synthetic diamond or a CVD synthesised singlecrystal diamond.

The quality of growth on a substrate which is not substantially free ofdefects rapidly degrades as the layer grows thicker and the defectstructures multiply, causing general crystal degradation, twinning andrenucleation. Defects of this type are particularly deleterious to thepresent application, causing local variations in many of the keyproperties.

The defect density is most easily characterised by optical evaluationafter using a plasma or chemical etch optimised to reveal the defects(referred to as a revealing plasma etch), using for example a briefplasma etch of the type described below. Two types of defects can berevealed:

-   -   1) Those intrinsic to the substrate material quality. In        selected natural diamond the density of these defects can be as        low as 50/mm² with more typical values being 10²/mm², whilst in        others it can be 10⁶/mm² or greater.    -   2) Those resulting from polishing, including dislocation        structures and microcracks forming chatter tracks along        polishing lines. The density of these can vary considerably over        a sample, with typical values ranging from about 10²/mm², up to        more than 10⁴/mm² in poorly polished regions or samples.

The preferred low density of defects is such that the density of surfaceetch features related to defects, as described above, are below5×10³/mm², and more preferably below 10²/mm².

The defect level at and below the substrate surface on which the CVDgrowth takes place may thus be minimised by careful preparation of thesubstrate. Included here under preparation is any process applied to thematerial from mine recovery (in the case of natural diamond) orsynthesis (in the case of synthetic material) as each stage caninfluence the defect density within the material at the plane which willultimately form the substrate surface when preparation as a substrate iscomplete. Particular processing steps may include conventional diamondprocesses such as mechanical sawing, lapping and polishing (in thisapplication specifically optimised for low defect levels), and lessconventional techniques such as laser processing, reactive ion etching,ion beam milling or ion implantation and lift-off techniques,chemical/mechanical polishing, and both liquid chemical processing andplasma processing techniques. In addition, the surface R_(Q) measured bystylus profilometer, preferably measured over 0.08 mm length) should beminimised, typical values prior to any plasma etch being no more than afew nanometers, i.e. less than 10 nanometers. R_(Q) is the root meansquare deviation of surface profile from flat (for a Gaussiandistribution of surface heights, R_(Q)=1.25 R_(a): for definitions, seefor example “Tribology: Friction and Wear of Engineering Materials”, IMHutchings, (1992), Publ. Edward Arnold, ISBN 0-340-56184).

One specific method of minimising the surface damage of the substrate isto include an in situ plasma etch on the surface on which thehomoepitaxial diamond growth is to occur. In principle this etch neednot be in situ, nor immediately prior to the growth process, but thegreatest benefit is achieved if it is in situ, because it avoids anyrisk of further physical damage or chemical contamination. An in situetch is also generally most convenient when the growth process is alsoplasma based. The plasma etch can use similar conditions to thedeposition or diamond growing process, but with the absence of anycarbon containing source gas and generally at a slightly lowertemperature to give better control of the etch rate. For example, it canconsist of one or more of:

-   -   (i) an oxygen etch using predominantly hydrogen with optionally        a small amount of Ar and a required small amount of O₂. Typical        oxygen etch conditions are pressures of 50-450×10² Pa, an        etching gas containing an oxygen content of 1 to 4 percent, an        argon content of 0 to 30 percent and the balance hydrogen, all        percentages being by volume, with a substrate temperature        600-1100° C. (more typically 800° C.) and a typical duration of        3-60 minutes.    -   (ii) a hydrogen etch which is similar to (i) but where the        oxygen is absent.    -   (iii) alternative methods for the etch not solely based on        argon, hydrogen and oxygen may be used, for example, those        utilising halogens, other inert gases or nitrogen.

Typically the etch consists of an oxygen etch followed by a hydrogenetch and then moving directly into synthesis by the introduction of thecarbon source gas. The etch time/temperature is selected to enableremaining surface damage from processing to be removed, and for anysurface contaminants to be removed, but without forming a highlyroughened surface and without etching extensively along extended defectssuch as dislocations which intersect the surface and thus cause deeppits. As the etch is aggressive, it is particularly important for thisstage that the chamber design and material selection for its componentsbe such that no material is transferred by the plasma from the chamberinto the gas phase or to the substrate surface. The hydrogen etchfollowing the oxygen etch is less specific to crystal defects roundingoff the angularities caused by the oxygen etch which aggressivelyattacks such defects and providing a smoother, better surface forsubsequent growth.

The primary surface of the diamond substrate on which the CVD diamondgrowth occurs is preferably the {100} surface. Due to processingconstraints, the actual sample surface orientation can differ from thisideal orientation up to 5°, and in some cases up to 10°, although thisis less desirable as it adversely affects reproducibility.

It is also important in the method of the invention that the impuritycontent of the environment in which the CVD growth takes place isproperly controlled. More particularly, the diamond growth must takeplace in the presence of an atmosphere containing substantially nocontaminants other than the intentionally added nitrogen. This additionof nitrogen should be accurately controlled, with an error of less than300 parts per billion (as a molecular fraction of the total gas volume)or 10% of the target value in the gas phase, whichever is the larger,and preferably with an error of less than 200 parts per billion (as amolecular fraction of the total gas volume) or 6% of the target value inthe gas phase, whichever is the larger, and more preferably with anerror of less than 100 parts per billion (as a molecular fraction of thetotal gas volume) or 3% of the target value in the gas phase, and evenmore preferably with an error of less than 50 parts per billion (as amolecular fraction of the total gas volume) or 2% of the target value inthe gas phase, whichever is the larger. Measurement of absolute andrelative nitrogen concentration in the gas phase at concentrations inthe range 300 ppb-5 ppm requires sophisticated monitoring equipment suchas that which can be achieved, for example, by gas chromatographydescribed in WO 01/96634.

The source gas may be any known in the art and will contain acarbon-containing material which dissociates producing radicals or otherreactive species. The gas mixture will also generally contain gasessuitable to provide hydrogen or a halogen in atomic form, and a sourceof nitrogen which may be for example N₂ or NH₃.

The dissociation of the source gas is preferably carried out usingmicrowave energy in a reactor, examples of which are known in the art.However, the transfer of any impurities from the reactor should beminimised. A microwave system may be used to ensure that the plasma isplaced away from all surfaces except the substrate surface on whichdiamond growth is to occur and its mount. Examples of preferred mountmaterials include molybdenum, tungsten, silicon and silicon carbide.Examples of preferred reactor chamber materials include stainless steel,aluminium, copper, gold and platinum.

A high plasma power density should be used, resulting from highmicrowave power (typically 3-60 kW, for substrate diameters of 25-300mm) and high gas pressures (50-500×10² Pa, and preferably 100-450×10²Pa).

Specific properties of diamond may also be enhanced by annealing, andthere is particular advantage in combining the technique of annealingwith the diamond of the invention, to obtain the widest range ofimproved properties. By annealing is meant any process in which elevatedtemperature is used in a controlled manner to bring about a beneficialmodification to any of the properties of diamond, either to thoseproperties detailed in this specification or to properties which inapplication are complementary to those properties. The heat treatmentwill vary according to the nature of the as-grown CVD diamond and thedesired changes to be produced. Properties of the diamond that are mostsensitive to annealing include optical scatter and (low) luminescence,although other properties such as birefringence and mechanical designstrength may also be improved. Annealing processes presumably furtherreduce local points of strain in the diamond as well as modifyingregions of non-diamond structure. Annealing may be near atmosphericpressure or at high pressure and typically takes place in thetemperature range above 1200° C. and more typically above 1700° C. Anupper limit on the annealing temperature range of 2500° C.-2800° C. isset by the limitation of current experimental capabilities althoughbenefit is anticipated from higher temperatures. Furthermore, annealingof CVD diamond in both the diamond and the graphite stable region hasbeen shown to reduce the absorption centres in diamond, enhancingoptical transmission, as described in co-pending internationalapplication PCT/IB03/03783, which can be of benefit.

A further important element is that annealing the diamond, andparticularly the diamond of the invention, reduces the luminescenceobserved from the diamond under certain conditions. In particular, wherethe diamond is being used as an optical window, luminescence from thewindow can mask the irradiation which the window is intended to giveaccess to. For example, where diamond is used as an anvil material whichprovides optical access to the sample under compression testing,luminescence from the anvil can be a severe limitation on the ability tostudy the optical characteristics of the material under compression. Aparticular non-limiting example is the luminescence from the 575 nm and637 nm centres. CVD synthetic diamond grown with significant nitrogenconcentrations in the starting gases or otherwise present in the processwill show luminescence from nitrogen-vacancy centres. The neutral andnegatively charged nitrogen-vacancy centres have zero-phonon lines at575 nm and 637 nm, respectively. Luminescence from both of these centresmay be excited with a 514 nm argon ion laser or other relatively shortwavelength radiation sources, and if strong would be a significantdisadvantage in using such single crystal CVD diamond in anvilapplications. The intensity of nitrogen-vacancy luminescence can besignificantly reduced by annealing treatment that dissociates thenitrogen-vacancy centres, for example by annealing at temperaturesaround and above 1800° C., using high pressure high temperatureannealing for higher annealing temperatures. By way of example, it hasbeen found that high pressure high temperature annealing at 1800° C. and75 kBars for 24 hours can substantially reduce the luminescence at 575nm and 637 nm.

Optical absorption at low levels is best measured by calorimetric means.Previous calorimetric measurements of optical absorption at 10.6 μm havebeen reported for polycrystalline CVD diamond layers (SE Coe et al,Diamond and Related Materials, Vol. 9, (2000) 1726-1729, and CSJ Pickleset al, Diamond and Related Materials, Vol. 9, (2000), 916-920).Typically absorption values at 10.6 μm in high quality optical gradepolycrystalline diamond fall in the range of absorption coefficientα=0.03 cm⁻¹-0.07 cm⁻¹, typical values being about 0.048 cm⁻¹.Measurement of natural diamond selected for low absorption is alsoreported to give a value of about 0.036 cm⁻¹. The similar lower limitseen in single crystal natural diamond and polycrystalline CVD diamondhas been attributed to the tail of the two phonon absorptions in thisregion, and thus has been considered as a fundamental limit.

It is thus surprising that the diamond of this invention can exhibit alower absorption coefficient of 0.0262 cm⁻¹, illustrating that even innatural diamond selected for low absorption there is a significantextrinsic absorption remaining at 10.6 μm which has not previously beenrecognized.

calorimetric measurements of diamond at 1.064 nm are less well reportedthan those at 10.6 μm, but a typical value for optical gradepolycrystalline diamond is absorption coefficient α=0.119 cm⁻¹. Incontrast, diamond made by the method of this invention has achievedvalues of α=0.0071 cm⁻¹. Such a low absorption coefficient makes thisdiamond particularly suited to high power laser applications and thelike. This is particularly the case when the low beam distortion fromthe low strain in the material is also considered.

Applications arising from the CVD diamond material of the invention,where performance is enabled by these unique material properties,include but are not limited to:

-   -   optical windows—for example where very high image quality is        required. The consistently high mechanical strength of the        material assists in designing for applications where the window        is pressurised.    -   laser windows—where high intensity beams need to pass        undisturbed through a window providing a degree of isolation. It        is particularly important that the laser beam does not interact        with the window in a manner which degrades the beam, for example        by local absorption and thermally induced strains, or cause        sufficient energy to be absorbed for the window to be        permanently damaged.    -   optical reflectors—where a surface needs to be extremely flat or        have a very accurately prescribed surface shape and be stable.    -   optical refractors and lenses—where one or both surfaces of an        optical transmission component are at least in part deliberately        non planar or non parallel, but must be manufactured to great        precision.    -   diffractive optical elements—e.g. where a structure in or on the        diamond is used to modify a light beam by diffraction.    -   etalons.    -   ornamental use, such as in watch glasses, or as gemstones.    -   anvils for high pressure high temperature experiments—in this        application the diamond may preferably be annealed.

For convenience, and by way of an example, the application of thediamond material of the invention to etalons will be described indetail, but those skilled in the art will recognise the generalimportance of the optical properties of the CVD diamond material of thisinvention to other applications such as those indicated above.

An optical system with two partially reflecting surfaces that has beenfabricated in such a way as to have a high degree of flatness andparallelism between the two reflecting surfaces is called a Fabry-PerotEtaIon. Typically the etalon can be made by aligning two very flatpartially reflective mirrors such that their reflecting surfaces areparallel and separated by, for instance, an air or controlled gaseousmedium gap or a vacuum separation. Alternatively the etalon can be madeby polishing two very parallel surfaces 10,12 onto a plate 14 of anoptically transparent solid material, called a solid etalon, as depictedschematically in FIG. 1 of the accompanying drawings.

A beam incident on the first surface of the etalon is partiallytransmitted and reflected according to the reflectivity of the surface.The transmitted beam traverses the etalon and subsequently at the secondsurface is partially transmitted and partially reflected back to thefirst surface where again partial transmission and reflection takeplace. As a result interference takes place between transmitted andreflected parallel beams emerging from the etalon. A typicaltransmission characteristic from an etalon is shown graphically in FIG.2.

The thickness of the etalon controls the separation of subsequentmaxima/minima of the etalon characteristic, known as the free spectralrange FSR, which for normally incident light is given below in terms offrequency,

$\begin{matrix}{{FSR} = \frac{c}{2n\; d}} & (1)\end{matrix}$where c is the speed of light in vacuum, d is the thickness of theetalon, and n is the refractive index of the etalon material.

The shape of the transmission curve (e.g. the sharpness of the peaksand/or the depth of the minima) is further influenced by thereflectivity of the etalon surfaces. Different values of thereflectivity may be obtained by applying partially reflecting opticalcoatings to the etalon surfaces, as is well known in the art.Alternatively one can choose not to apply optical coatings to the etalonsurfaces and use the Fresnel reflectivity of the uncoated surfaces ofthe etalon.

When the etalon transmission curve shows sharp peaks this may becharacterised by the finesse, F, defined as the ratio of the (frequency)spacing between successive peaks over the full-width-half-maximum of thepeaks. For high values of the reflectivity and when losses due toabsorption or scatter in the etalon or at the reflecting surfaces anddeviations from flatness and parallelism of the reflecting surfaces areso small they can be neglected, the finesse is given by:

$\begin{matrix}{F = \frac{\pi\sqrt{R}}{\left( {1 - R} \right)}} & (2)\end{matrix}$where R is the reflectivity of the etalon surface.

Alternatively, when transmission peaks are not very sharp, one cancharacterise the etalon transmission curve by specifying the contrastratio, C. This is given by the ratio of the maximum and minimumtransmission values,

$\begin{matrix}{C = \frac{T_{p}}{T_{v}}} & (3)\end{matrix}$where T_(p) (T_(v)) is the transmission of the etalon at a frequencyequal to one of the peaks (valleys) in the transmission curve.

For etalons where deviations from flatness or parallelism, refractiveindex variations and absorption or scatter losses can be neglected, C isgiven by

$\begin{matrix}{C = {{1 + \frac{4\; R}{\left( {1 - R} \right)^{2}}} = \left( \frac{1 + R}{1 - R} \right)^{2}}} & (4)\end{matrix}$

Another useful parameter to characterise the etalon performance is theinsertion loss, L, expressed in decibel (dB), which is determined by thetransmission of the etalon at the peaks,

$\begin{matrix}{L = {{{- 10}x^{10}{\log\left( T_{p} \right)}} = {{- 10}x^{10}{\log\left( \frac{I_{p}}{I_{0}} \right)}}}} & (5)\end{matrix}$where I_(p) and I₀ are the transmitted and incident intensities at afrequency equal to one of the peaks in the transmission curve. Thusdefined the insertion loss can vary between 0 (no loss) and infinite (notransmission at all). For an ideal etalon without losses and withinfinitely flat and parallel surfaces, the insertion loss would be 0.

When deviations from flatness or parallelism, refractive indexvariations or losses cannot be neglected, the approximate equations (2)and (4) are no longer valid and insertion loss (5) will tend to increasewhile the contrast ratio generally will be lower.

A variant of the Fabry-Perot etalon is the Gires-Tournois etalon whichis essentially a Fabry-Perot etalon used in reflection with reflectivityof the back surface being 100%. The reflectivity of such an etalon isalways 100%, independent of the wavelength of the incoming light but thephase of the reflected light is a periodic function of the incominglight frequency with a periods equal to that of the FSR, as defined inequation, of the Gires-Tournois etalon. This is shown in FIG. 3, where δis defined as

$\delta = {{4\;\pi\frac{nd}{c}f} = {2\;\pi\frac{f}{FSR}}}$

where f is the frequency.

The important material properties influencing etalon performance,expressed by the parameters free spectral range, insertion loss,contrast ratio and/or finesse, are thus:

R—surface reflectance (either intrinsic when uncoated or of thecoating);

α—absorption losses in the bulk of the etalon material or at thesurface;

α_(sc)—scatter losses in the bulk of the etalon material or at thesurface;

n—refractive index of the etalon material and variations in it(including birefringence, i.e. dependence of the refractive index onpolarisation and propagation direction in the material); and

d—flatness and parallelism of the reflecting surfaces.

Diamond has a number of advantages when used as an etalon compared withother materials, including:

-   -   a) a high refractive index, which translates into a more        compact/thinner etalon;    -   b) a Fresnel reflectivity which in some applications is high        enough so as to make optical coatings unnecessary;    -   c) a low temperature coefficient of refractive index and a low        thermal expansion coefficient, which means that diamond etalons        are less sensitive than some other optical materials to        temperature changes;    -   d) a high thermal conductivity, which means that there is        minimal variation in the transmission curve caused by        temperature variations in the environment or absorption by the        light beam (further increases in thermal conductivity are,        however, beneficial for this reason);    -   e) the high strength and stiffness, relative to other materials,        and high hardness of diamond, which makes it strong and        impervious to scratching (if uncoated)—it also minimises the        effects of any mounting induced stresses.

However, the use of diamond as an etalon material has been very limited.The limitation has been the availability of material with suitableproperties, particularly those properties that are sensitive to thediamond quality, and in suitable sizes. For example, the most abundantnatural diamond is type Ia. Type Ia natural diamonds are generallylimited in size, and their price determined by their use as gemstones.Material available for commercial applications is mostly faint yellowcoloured (absorbing), contains inclusions (stressed, scattering), andalso contains hydrogen, which may give rise to further absorptions inthe visible and infrared ranges of the spectrum. The refractive indexvariations between natural stones can be as high as 1%. Functionality inthe intended application can only be assured by costly screening of eachpiece of material, which typically can only be performed aftersubstantial processing has taken place.

The CVD diamond material of the invention provides a material, superiorto other diamond and other materials, as an etalon material, such as ina Fabry-Perot etalon or a Gires-Tournois etalon.

The CVD single crystal diamond material of the invention, as described,has one or more key characteristics. Some of these characteristics andthe techniques which may be used to measure or determine them will nowbe described.

Optical Characteristics and Measurement Techniques

Optical Homogeneity

The optical homogeneity was measured using a ZYGO GPI phase shifting 633nm laser Fizeau-type interferometer. Samples were typically prepared asoptical plates 0.8 and 1.25 mm thick and up to 5 mm×5 mm lateraldimensions with flat polished surfaces. Measurements were made using a4% reflectivity flat, beam splitter and combining the reflected beamfrom this beam splitter with the transmitted beam after dual passagethrough the diamond plate with an intermediate reflection off a 10%reflective flat mirror. Both the beam splitter and the reflective mirrorwere of interferometric quality with flatness better than approximately30 nm over their diameters of 100 mm. The resulting interference patternwas recorded with a charge coupled device (CCD) camera and digitallystored. The interference fringes were then analysed using theTransmitted Wavefront Measurement Application module which is suppliedas standard software with the Zygo GPI interferometer. Deviations from aperfectly flat wavefront were thus recorded. These deviations are acombination of the effects of non-flatness of the surfaces and opticalnon-homogeneity of the diamond material. By polishing the surfaces tohigh enough flatness (better than 30 nm) the effects of thenon-homogeneity could be determined to better than 0.05 fringe,proportionately lower levels of flatness being permissible for lessaccurate measurements.

Effective Refractive Index

The effective refractive index was measured by first measuring thethickness of an optical plate processed in the shape of an etalon with adigital micrometer with resolution better than 0.5 μm and then measuringthe Free Spectral Range of the etalon over the frequency range of 197THz-192 THz using light that is perpendicularly incident on the etalon,such that the required accuracy in the effective refractive index couldbe obtained. The effective refractive index was then found from equation(1) defined earlier. The effective refractive index found by this methodcan differ slightly from the refractive index found for example bysimple application of Snell's law (refraction of light at the interfacebetween two optical media), the value obtained here generally beinghigher. The difference arises because of the inevitable dispersionpresent in the diamond, and the fact that the method used here for theeffective refractive index is a form of average obtained from the rangeof frequencies used in the measurement.

Free Spectral Range (FSR)

FSR was measured for a plate suitably processed in the form of an etalon(e.g. 1.5 mm×1.5 mm in the lateral dimensions and 1.25 mm thick, withpeak-to-valley surface flatness better than 40 nm, as measured with aZygo-NewView interferometer, using the flatness application included inthe software of the Zygo-NewView interferometer, and parallelism of thepolished surfaces better than 10 arcsec and surface roughness R_(a)better than 1 nm). These plates were mounted on a optical stage withtranslational and rotational capability along two mutually perpendicularaxes in the plane of the diamond etalon. The etalon was then positionedperpendicular to and centered with respect to a collimated beam from alaser diode whose wavelength can be continuously varied between 1.52 and1.62 μm. The power transmitted through the etalon as a function of thefrequency of the light was recorded and stored in digital form in acomputer. From the frequency difference between successive peaks in thetransmission spectrum the Free Spectral Range was directly determined.

Contrast Ratio and Insertion Loss

Contrast Ratio and Insertion Loss were measured for a plate suitablyprocessed into the form from which discrete etalons can be cut (e.g. 4.0mm×4.0 mm in the lateral dimensions and 1.25 mm thick), withpeak-to-valley surface flatness better than 40 nm, as measured with aZygo-NewView interferometer, using the flatness application included inthe software of the Zygo-NewView interferometer, and parallelism of thepolished surfaces better than 10 arcsec and surface roughness Ra betterthan 1 nm. These plates were mounted on an optical stage withtranslational and rotational capability along two mutually perpendicularaxes in the plane of the diamond etalon. The plate was then positionedperpendicular to and centered with respect to a collimated beam from alaser diode whose wavelength can be continuously varied between 1.52 and1.62 μm. The power transmitted through the plate as a function of thefrequency of the light was recorded and stored in digital form in acomputer. The contrast ratio of each etalon was determined bycalculating the ratio of the measured maximum and minimum transmissionat a frequency of about 197200 GHz.

Birefringence

For an isotropic medium, such as stress-free diamond, the refractiveindex is independent of the direction of the polarization of light. If adiamond sample is inhomogeneously stressed, either because of grown-instress or local defects or because of externally applied pressure, therefractive index is anisotropic. The variation of the refractive indexwith direction of polarization may be represented by a surface calledthe optical indicatrix that has the general form of an ellipsoid. Thedifference between any two ellipsoid axes is the linear birefringencefor light directed along the third. This may be expressed as a functioninvolving the refractive index of the unstressed material, the stressand opto-elastic coefficients.

The Deltascan (Oxford Cryosystems) gives information on how therefractive index at a given wavelength depends on polarization directionin the plane perpendicular to the viewing direction. An explanation ofhow the Deltascan works is given by A. M. Glazer et al. in Proc. R. Soc.Lond. A (1996) 452, 2751-2765.

From a series of images captured for a range of different relativeorientations of a pair of plane polarising filters the Deltascandetermines the direction of the “slow axis”, the polarization directionin the plane perpendicular to the viewing direction for which therefractive index is a maximum. It also measures |sin δ| where δ is thephase shift given byδ=(2π/λ)ΔnLwhere λ is the wavelength of the light, L is the thickness of thespecimen and Δn is the difference between the refractive index for lightpolarized parallel to the slow and fast axes. Δn L is known as the‘optical retardation’.

For retardation in first order, with L=0.6 mm and λ=589.6 nm, then:

when sin δ=1 and Δn L=λ/4, it can be deduced that Δn=2.45×10⁻⁴.

when sin δ=0.5 and Δn L=λ/12, it can be deduced that Δn=0.819×10⁻⁴.

The Deltascan produces three colour-coded images showing the spatialvariations of a) the “slow axis”, b) sin δ and c) the absorbance at thewavelength of operation.

Samples are prepared as optical plates of known thickness and analysedover an area of at least 1.3 mm×1.3 mm, and preferably 2.5 mm×2.5 mm,and more preferably 4 mm×4 mm. Sets of Deltascan images or ‘frames’,each covering an area of 1 mm×0.75 mm, are recorded for each sample at awavelength of 589.6 nm. Within each frame, the Deltascan individuallyanalyses 640×480 pixels, ensuring the sample is analysed at very finescale. The array of Deltascan |sin δ| images is then analysed for thebehaviour of sine δ. The simplest analysis is to identify the maximumvalue of sine δ in each 1 mm×0.75 mm frame over the whole of theanalysis area and use these values to characterise the maximum value ofthe whole of the area analysed. Where the array of 1 mm×0.75 mm framesdoes not exactly match the area under analysis, the frames are arrangedto obtain the minimum total number of frames to entirely cover the area,and centred so as to make utilisation of edge frames as symmetric aspractical. That part of the data in any frame which is from outside theboundary of the area under analysis is then excluded from the analysisof that frame. Alternatively, each 1 mm×0.75 mm frame can be analysedfor the maximum value remaining after exclusion of 2%, or 1% of the datawithin it that lies within the analysed sample area, so as to obtain themaximum value over 98%, or 99% respectively of the material of the areaanalysed. This may be relevant where the application can tolerate a fewisolated points of higher birefringence. However, in all the examplesgiven in this specification all datapoints (100%) have been included inthe analysis.

The behaviour of sine δ is the property of a particular plate ofmaterial, constrained here to plates of useful thickness by applicationof a minimum thickness. A more fundamental property of the material canbe obtained by converting the sine δ information back to a valueaveraged over the thickness of the sample of the difference between therefractive index for light polarised parallel to the slow and fast axes,Δn_([average]).

Optical Absorption

Optical absorption is measured by laser calorimetry, with a thermocoupleattached to the sample under test to measure the rise in sampletemperature resulting from the passage through the sample of the laserbeam of the required wavelength. Such techniques are well known in theart. In particular the methods used here conform to the InternationalStandard ISO 11551:1997(E) and were made at 1.064 μm and 10.6 μm.

Optical Scatter

Methods for the measurement of optical scatter are well known (see forexample DC Harris, “Infrared Window and Dome Materials”, SPIE,Washington, USA 1992). However, in diamond pieces of small size (e.g.4×4 mm laterally) and of the quality made possible by this method, ithas been found necessary to develop a new technique for measuringscatter precisely.

The new technique has been developed for measurement primarily at 1.06μm, although other wavelengths such as 633 nm are possible. A diagram ofthe experimental set up for the method is shown in FIG. 4.

A 1.06 μm Nd-YAG laser 20 illuminates the sample 22 and the scatteredbeam 24 is detected through a defined aperture 26 with a highly linear,wide dynamic range detector 28. The sample 22 and detector 28 aremounted on separate goniometer stages (not shown) allowing precisemovement of each. The whole system is in a class 100 “clean tent” (notshown) to minimise scatter by atmospheric dust and the clean tent itselfis in a dark room to avoid stray light affecting the results.

The detector 28 is moved on an arc 30 from −85° to +85° in 5° steps,except in the region close to the through beam where movement is in 1°steps. Measurements are made with the incoming beam polarised eitherhorizontally or vertically. The solid angle of the detector is 0.741mSr. The goniometer stage for the diamond sample 22 allows it to beaccurately aligned with respect to the incoming beam and then fixed forthe duration of the experiment. The diagram of FIG. 4 is not to scale,with the distance between the sample and the detector being 400 mm. Theunscattered beam is indicated by reference numeral 32.

The data from the detector is gathered in a series of discrete steps andtherefore represents a histogram of the scatter as a function of angle,measured over the restricted solid angle of the detector. This data isconverted from essentially a one-dimensional array covering a strip froma hemisphere into two-dimensional data covering bands around ahemisphere. This is illustrated in FIG. 5, where 34 is the scatteredbeam as measured and 36 is the converted data to give the totalintensity scatter at this angle, as described more fully below.

The data gathered is defined as the fractional scattered power per unitsolid angle at angle θ (“FSP(θ)”):

$\frac{P_{m,\theta}}{P_{i}\Omega}{Sr}^{- 1}\mspace{14mu}{at}\mspace{14mu}{angle}\mspace{14mu}\theta$where P_(m,θ) is the measured power at the detector at angle θ, P_(i) isthe incident beam power and Q is the solid angle of the detector.

Light scattered by angle θ goes into a band with circumference 2πR sinθ, height RΔθ and therefore area, A_(r)=πR sin θ. RΔθ.

The solid angle of the band, ΔΩ_(r), is given by

${\Delta\Omega}_{r} = {\frac{A_{r}}{R^{2}} = {2\;\pi\mspace{11mu}\sin\;\theta\;\Delta\;\theta}}$

From this, the total scattered power, TSP, the value we require is givenby:

TSP = ∫₀^(π/2)FSP(θ) ⋅ ΔΩ_(r)

However, the data is discrete rather than continuous so the TSP must becalculated by a summation rather than an integration operation:

${TSP} = {\sum\limits_{0}^{\pi/2}\;{\left( \frac{{vp} + {hp}}{2} \right)_{\theta}2\;\pi\mspace{11mu}\sin\;{\theta\Delta\theta}}}$in which vp and hp are the vertically and horizontally polarised dataand AO is the data interval radians. Care has to be taken with the datainterval as this varies, being 1° at low angles and 5° for angles morethan 5° from the unscattered beam.Laser Damage Threshold

The laser damage threshold is measured by firing pulses of a laser atthe sample under test, and characterising the point of failure as themean of the lowest incident peak energy that causes damage and thehighest incident peak energy that does not cause damage.

At a wavelength of 10.6 μm a CO₂ laser was used with a primary spike ofthe order of 50-100 ns containing typically ⅓ of the total pulse energy,and a much lower peak power relaxation pulse of the order of 2 μs. at aobtained was normalised to a 100 μm 1/e spot size. The relaxation pulsecan be neglected because this test operates in the time domain whereelectron avalanche ionisation is the conventional model for damage tooccur, which is thus dependent on peak power density (i.e. peak electricfield).

At a wavelength of 1.06 μm a Nd:YAG laser was used with a single spikeof 10-50 ns duration, and more preferably in the range 20-40 ns, withdata again normalised to a 100 μm 1/e spot size.

Thermal Conductivity

Thermal conductivity is measured by the laser flash technique (see forexample DJ Twitchen et al., Diamond and Related Materials, 10 (2001) p731, and CJH Wort et al, Diamond and Related Materials, 3 (1994) p1158).

Surface Roughness

Surface roughness was measured using a Zygo NewView 5000 scanning whitelight interferometer. The interferometer utilises a microscope equippedwith an interferometric objective of the Michelson or the Mireau type.Magnifications of between 1× to 50× are possible with this system. Bymeasuring over the full area of the diamond plate we found that surfaceroughness varied by less than 10% over the area of the plate if it wasfully fine-polished. Therefore in the current measurements the roughnesswas inferred from measurement over a representative area of approx. 0.36mm×0.27 mm.

Surface Parallelism

Surface parallelism was measured using a Zygo GPI phase shifting 633 nmlaser Fizeau-type interferometer in a set-up identical to themeasurement of the transmitted wavefront. By comparing the transmittedwavefront fringe pattern with the diamond etalon in the beam path withthe pattern measured without an etalon in the beam path, the change indirection of and distance between successive fringes was computed andfrom this the deviation from parallelism between the two polishedsurfaces of the etalon was determined. These two fringe patterns weremeasured simultaneously by passing part of the light through the etalonwhile at some other position the light was directly incident on the 10%reflective flat mirror and was again reflected back towards the detectorwithout passing through the diamond etalon. The simultaneous measurementallowed for higher accuracy than if the two measurements were performedone after the other.

Surface Flatness

Surface flatness was measured using a Zygo GPI phase shifting 633 nmlaser Fizeau-type interferometer. With this interferometer the lightfrom a 633 nm laser source was partially reflected from a 10%reflectivity, interferometric quality beam splitter and the lighttransmitted by the beam splitter was partially reflected by the polishedsurface of a diamond optical component manufactured from the material ofthe invention. The two reflected beams were combined and the resultingfringe pattern was recorded with a CCD camera detector and storeddigitally in a computer. The pattern was subsequently analysed with theflatness application which is included as a standard application in thesoftware of the Zygo GPI interferometer.

Mechanical Strength

The utility of the material of this invention is clearly illustrated bythe absence of reported strength data in single crystal diamond whichhas been obtained by actual fracture tests. Data currently reported arebased on indentation tests, and the approximations and assumptions whichare inherent in this approach. Conversely, the method of this inventionmakes material available in sufficient quantity that proper fracturetests can be completed.

Furthermore, fracture strength testing is a destructive test. Since eachpiece of natural diamond is unique, once its strength is known then itis no longer available for application. Fracture testing can then onlybe used to characterise the spread of strength against some proxycharacteristic, and the lowest anticipated strength used forapplication. In contrast, the synthetic diamond of the invention is awell characterised and consistent material such that the fracturestrength of a particular element can be reasonably predicted based onfracture statistics of equivalent samples. The design strength ofdiamond, as used in this specification, is the strength which isexhibited by at least 70%, preferably 80%, and more preferably at least90% of equivalent samples of material tested using the procedure below.

The strength was measured using a single cantilever beam technique, witha sample size of 5.0 mm by 3.0 mm by 0.18-0.35 mm (length, l, bybreadth, b, by thickness, d). The samples were cut from {100} orientedplates, with the long axis along the <110> direction (so that thethickness is along the <100> and the length and breadth are along the<110>). The testing procedure mounted the beams with an exposed lengthof 4 mm (i.e. 1 mm inside the clamp) and applied the force at a distanceof 3.5 mm from the clamp.

The strength, σ_(b), is given by the expression:σ_(b)=(6Ws)/(bd ²)where W is the breaking load and s is the distance between the loadingline and the clamping line.

Test samples were cut from homoepitaxial CVD diamond plates andcarefully prepared by scaife polishing on progressively finer and finergrits down to a grit size of approximately 0.1 μm. Poor surface finishcan limit the measured strength of the material and the ability of thismaterial to take a high surface finish may contribute to its overallstrength.

Luminescence

Quantitative luminescence properties of diamond samples can be obtainedby normalising the integrated intensities of the relevant luminescencelines or bands relative to the integrated intensity of the diamond Ramanscattering collection under the same conditions. The measurements aremade at 77K with a 300 mW 514 nm argon ion laser beam and spectrarecorded using a Spex 1404 spectrometer equipped with a holographicgrating (1800 grooves/mm) and a Hamamatsu R928 photomultiplier. The datais corrected to allow for spectral response function of the spectrometersystem, derived using a standard lamp with a known spectral output.

The invention will now be discussed in further detail by way of thefollowing non-limiting examples.

Example 1

Substrates suitable for synthesising single crystal CVD diamond of theinvention may be prepared as follows:

-   -   i) Selection of stock material (type Ia natural stones and type        Ib HPHT stones) was optimised on the basis of microscopic        investigation and birefringence imaging to identify substrates        which were free of strain and imperfections.    -   ii) Laser sawing, lapping and polishing to minimise subsurface        defects using a method of a revealing plasma etch to determine        the defect levels being introduced by the processing.    -   iii) After optimisation it was possible routinely to produce        substrates in which the density of defects measurable after a        revealing etch is dependent primarily on the material quality        and is below 5×10³/mm², and generally below 10²/mm². Substrates        prepared by this process are then used for the subsequent        synthesis.

A high temperature/high pressure synthetic type 1 b diamond was grown ina high pressure press, and as a substrate using the method describedabove to minimise substrate defects to form a polished plate 5 mm×5 mmsquare by 500 μm thick, with all faces {100}. The surface roughnessR_(Q) at this stage was less than 1 nm. The substrate was mounted on atungsten substrate using a high temperature diamond braze. This wasintroduced into a reactor and an etch and growth cycle commenced asdescribed above, and more particularly:

-   -   1) The 2.45 GHz reactor was pre-fitted with point of use        purifiers, reducing unintentional contaminant species in the        incoming gas stream to below 80 ppb.    -   2) An in situ oxygen plasma etch was performed using 15/75/600        sccm (standard cubic centimeter per second) of O₂/Ar/H₂ at        263×10² Pa and a substrate temperature of 730° C.    -   3) This moved without interruption into a hydrogen etch with the        removal of the O₂ from the gas flow.    -   4) This moved into the growth process by the addition of the        carbon source (in this case CH₄) and dopant gases. In this        instance was CH₄ flowing at 36 sccm and 1 ppm N₂ was present in        the process gas, provided from a calibrated source of 100 ppm N₂        in H₂ to simplify control. The substrate temperature at this        stage was 800° C.    -   5) On completion of the growth period, the substrate was removed        from the reactor and the CVD diamond layer removed from the        substrate.

The CVD diamond layer grown above can be sufficiently large to produceat least one and preferably several diamond etalons (in a twodimensional array) depending on the size of the CVD diamond layer andthe required size of the etalons and sufficiently thick to prepare atleast one and preferably more CVD diamond plates for processing

After removal from the substrate the diamond layer grown as describedabove was sawn into a number of diamond plates (if required) and eachdiamond plate was subsequently polished to just above the desiredthickness of 1.25 mm, which is the thickness of the desired diamondetalon as defined by the required free spectral range and intendedwavelength of operation, using polishing techniques known in the art.

The plate was then fine polished one side on a cast iron diamondpolishing wheel that had been carefully prepared. The tang used was veryrigid and held the diamond against a reference surface that ran parallelto the scaife surface.

The diamond plate was then turned over and the other side was polishedto the desired flatness and parallelism on the same scaife, taking careat this stage to bring the thickness to that required for the finaletalon. Parallelism was measured using a commercial Zygo GPIinterferometric instrument based on the Fizeau principle, well known tothose skilled in the art. The thickness was measured initially by amicrometer, using measurement of the free spectral range (FSR) as afinal stage check. Final thickness was achieved by measuring the linearremoval rate, which because of the quality of the material was veryconstant, and then polishing for the necessary predicted time. Othermethods of etching or material removal have been used, including ionbeam etching, plasma etching or reactive ion etching.

The plate was then cut up by a laser into discrete units. The side faceswere then polished, although this is not always required by theapplication.

The resultant diamond etalon was 1.5 mm square, 1.251 mm thick, made tothe following tolerances:

-   -   thickness:—±0.25 μm    -   parallelism—: ±5 arcsec    -   surface R_(a):—0.5 nm    -   and had a FSR of 1.6678±2×10⁻⁴ cm⁻¹.

Another diamond plate from the above synthesis process was used tofurther characterise the achievable surface R_(a). The surface wascarefully polished on both sides as described above and then measuredfor surface R_(a) using the Zygo NewView 5000 scanning white lightinterferometer. Measurements were taken each side of the sample, eachmeasurement on a 1 mm×1 mm area with the 9 areas forming a 3 mm×3 mmgrid on the centre of each side, and then the statistical mean of the 9measurements was calculated. The measured R_(a) on side A was 0.53nm±0.04 nm, and on side B was 0.54 nm±0.05 nm.

Example 2

A set of 6 mm×6 mm×0.4 mm plates of homoepitaxial CVD diamond weresynthesised according to the method described in Example 1. From theseplates, a set of rectangular test samples, 3 mm×5 mm in lateraldimensions and 0.17 to 0.22 mm thick were cut, ensuring that the cutpieces were free from growth sector boundaries.

The set of samples was polished on a scaife using a range of diamondpowders down to 0.1 μm. Care was taken to ensure that, as far aspossible, all sub-surface damage was removed from the samples. The finalpolishing stage with the finest grit is vital as this controls the finalsurface flaw size distribution. After the top and bottom surfaces werepolished, the edges of the samples were prepared to the same standard.After polishing was complete, the surfaces were examined by Nomarskiinterference contrast and ‘micromapped’ to check the surface roughness.Nomarski microscopy at a magnification of ×200 revealed that there whereno visible defects in the surface. The surface roughness, as defined bythe R_(a) value, was determined using a non-contact, optical surfaceroughness measurement instrument (‘Micromap’). Two series of 200 μm longscans were made in perpendicular directions and the resulting R_(a)values were averaged yielding a mean R_(a) value of less than 0.25 nm.This compares with typical R_(a) values of between 1 nm and 5 nm fordiamonds polished using the same technique as is used for polishingnatural diamonds in the jewelry trade.

An additional stage of ion beam etching was applied to the surface ofsome of the samples prior to final polishing. A further optionaltechnique would be to chemically thin the samples prior to finalpolishing.

The strength of the plates was measured by single cantilever beambending. The individual strength values for a set of nine samplesapproximately 0.2 mm thick were, in GPa, 1.50, 1.63, 2.50, 3.26, 3.30,4.15, 4.29, 4.83, 5.12. Analysis of this and other datasets suggeststhat the two lowest values are from a different population to the otherseven, possibly indicating that the sample preparation was notsufficiently careful in this instance to avoid any influence on themeasured strength. Even with these two suspect data points included, 77%of samples have a breaking strength of at least 2.5 GPa, and the datasuggests the strength is actually in excess of 3 GPa.

For comparison, there being no equivalent data in the public domain (allknown strength measurements on natural diamond are based on indentationtesting, which is an indirect and less reliable method, because of therestricted availability of suitable samples), the strength of a batch offive type IIa natural diamond plates was also measured. These plateswere carefully selected by examination with an optical microscope at ×50magnification to be free of inclusions and other flaws which may weakenthe diamond, and were prepared and tested by the same technique. Theindividual strength values for this set of 5 samples approximately 0.18mm thick were, in GPa, 1.98, 2.08, 2.23, 2.61, 2.94 clearly limited bythe intrinsic properties of the material. Similarly type Ib singlecrystal diamond synthesised by a high pressure-high temperature processwere carefully selected, processed, and tested using the sametechniques. The individual strength values for this set of 14 samplesapproximately 0.35 mm thick were, in GPa, 0.94, 1.11, 1.16, 1.3, 1.35,1.38, 1.46, 1.50, 1.54, 1.6, 1.65, 1.72, 1.73, 1.98, 2.17.

The strength population of the CVD diamond of this invention is clearlydistinct and higher than that of either the natural or HPHT diamond.

A particular application of high strength diamond is in optical windowsfor gas analysis by infrared spectroscopy. A particular window, which is7 mm in diameter, has a clear aperture of 5 mm and is brazed around theouter 1 mm of one flat surface, has to withstand a pressure differentialof 200 atmosphere with a safety factor of 4.

The breaking strength is related to the thickness, t, by:t=√(3r ² Pk/8σ_(b))where r is the clear aperture, P, the pressure, σ_(b), the breakingstrength and k a constraint factor which, for diamond, is 3.1 forunconstrained at the edge and 1.1 for fully constrained at the edge(assuming Poissons ratio for diamond to have a value of 0.1). Becausedetermining the degree of constraint is difficult, we take theworst-case scenario of the edge being unconstrained.

If a natural diamond window (design strength 2.0 MPa) were used in thisapplication, the thickness would need to be 0.54 mm. With strong singlecrystal CVD diamond of the invention (design strength 3.0 MPa), thethickness could be reduced to 0.44 mm. The reduction in the thickness ofthe material will reduce the cost of the window.

Example 3

A set of 3 plates of homoepitaxial CVD diamond were synthesisedaccording to the method described in Example 1. These were prepared asoptical plates of thickness 0.60-0.64 mm and with lateral dimensions ofup to 6 mm×6 mm. Sets of Deltascan images, each covering an area of 1mm×0.75 mm, were recorded for each sample at a wavelength of 589.6 nm.

Each Deltascan sine δ image was analysed for the maximum value of |sinδ| using 100% of the data and theses maximum values are shown in the|sin δ| maps of FIGS. 10-12.

FIG. 10 is the Deltascan map of sample E4.1, showing the maximum valueof |sin δ| in each frame of 1 mm×0.75 mm.

Analysis of the data of FIG. 10 shows the following

over an area of 2.0 mm×2.25 mm the maximum value for |sin δ| is 0.3

over an area of 3.0 mm×4.0 mm the maximum value for |sin δ| is 0.6

over an area of 5.25 mm×4.0 mm the maximum value for |sin δ| is 0.9

FIG. 11 is the Deltascan map of sample E4.2, showing the maximum valueof |sin δ| in each frame of 1 mm×0.75 mm.

Analysis of the data of FIG. 11 shows the following

over an area of 2.0 mm×3.75 mm the maximum value for |sin δ| is 0.3

over an area of 3.0 mm×3.75 mm the maximum value for |sin δ| is 0.4

over an area of 4.0 mm×4.5 mm the maximum value for |sin δ| is 0.7

FIG. 12 is the Deltascan map of sample E4.3, showing the maximum valueof |sin δ| in each frame of 1 mm×0.75 mm.

Analysis of the data of FIG. 12 shows the following

over an area of 3.0 mm×2.25 mm the maximum value for |sin δ| is 0.2

over an area of 3.75 mm×3.0 mm the maximum value for |sin δ| is 0.6

over an area of 4.0 mm×4.5 mm the maximum value for |sin δ| is 0.9

Example 4

8 sets of homoepitaxial CVD diamond layers were synthesised according tothe method described in Example 1. The CVD diamond layers had lateraldimensions of up to 6.5 mm and thicknesses of up to 3.2 mm. From theseCVD diamond layers a total of 48 plates were prepared as etalon plateswith lateral dimensions of typically 4.0×4.0 mm and thicknesses ofapproximately 1.25 mm. The thickness of each plate was measured with amicrometer to an accuracy of better than ±0.25 μm.

The average FSR of each plate over the C-band (197200-192200 GHz) wasdetermined by measuring the frequencies of the peak position of theetalon transmission peak nearest to the start and end position of theC-band for perpendicular incident light. A 1 mm diameter beam was used.These peak positions could be determined to an accuracy better than ±0.5GHz. The effective FSR is calculated by dividing the frequency intervalbetween the two extreme peaks by the number of periods in thetransmission spectrum (For this etalon thickness and frequency bandtypically 100 periods). Thus the error in the determination of the FSRis better than ±10 MHz.

FIG. 6 shows the measured FSR as a function of the inverse of the samplethickness for each of the plates. From the slope of the least-squarestraight line fit to the data an effective refractive index wascalculated using equation (1) which gave a value for the averageeffective refractive index of:n _(eff,av)=2.39695with a standard deviation of

-   -   σ_(n)=0.00045

The maximum deviation from the average was:Δn _(max)=±0.00092

The maximum relative measured deviation is therefore found to be:

$\frac{\Delta\; n_{\max}}{n_{{eff},{av}}} = {{\pm 3.84} \times 10^{- 4}}$

This measured deviation is due to measurement errors in thickness d andFSR and due to the variation in the refractive index from sample tosample due to material inhomogeneity, Δn_(inhom)

$\frac{\Delta\; n_{\max}}{n_{{eff},{av}}} = {\left( \frac{\Delta\; n}{n} \right)_{meas} + \left( \frac{\Delta\; n_{{in}\mspace{11mu}\hom}}{n} \right)}$

Error analysis shows that the maximum relative measurement error in ndue to the measurement errors in thickness d and FSR is:

$\left( \frac{\Delta\; n}{n} \right)_{meas} = {{\frac{\Delta\; d}{d} + \frac{\Delta\;{FSR}}{FSR}} = {{\frac{0.25\mspace{14mu}{µm}}{1.255\mspace{14mu}{mm}} + \frac{10\mspace{14mu}{MHz}}{50\mspace{14mu}{GHz}}} = {{\pm 4.0} \times 10^{- 4}}}}$

The maximum relative deviation Δn_(max)/_(neff,av) is largely determinedby the measurement error in thickness (Δd) and FSR (ΔFSR).

An upper limit on the variation in the value of the refractive index,Δn_(inhom)/n, is therefore at least better than ±4×10−4.

Example 5

8 sets of homoepitaxial CVD diamond layers were synthesised according tothe method described in Example 1. The CVD diamond layers had lateraldimensions of up to 6.5 mm and thicknesses of up to 3.2 mm. From theseCVD diamond layers a total of 48 plates were prepared as etalon plateswith lateral dimensions of typically 4.0×4.0 mm and thicknesses ofapproximately 1.25 mm.

The contrast ratio of each etalon plate was determined as describedearlier and the results are plotted in FIG. 7, where the solid lineindicates the theoretically maximum value for an ideal etalon, for whichthe surface reflectivity is determined by the refractive index

${C_{th} = \left( \frac{1 + R}{1 - R} \right)^{2}},{R = \left( \frac{n - 1}{n + 1} \right)^{2}}$

(Note that here the effective refractive index is not the correct valueto use, but that the actual refractive index at the considered frequencyis to be used which has the value n=2.3856, as can be determined fromthe Sellmeier equation, describing the dispersion of the refractiveindex. This value is consistent with the measured value of the effectiverefractive index.) The theoretical value of the contrast ratio is:C _(th)=1.967

The average measured contrast ratio is:C _(meas,av)=1.89with a standard deviation:σ_(c)=±0.04

Any imperfection in etalon material properties (inhomogeneities in thevolume) and etalon preparation (imperfect parallelism, flatness,roughness of the surfaces) reduces the contrast ratio. The consistentlyhigh value of the contrast ratio demonstrates the homogeneity of thematerial properties and the accuracy of the etalon preparation.

Example 6

8 sets of homoepitaxial CVD diamond layers were synthesised according tothe method described in Example 1. The CVD diamond layers had lateraldimensions of up to 6.5 mm and thicknesses of up to 3.2 mm. From theseCVD diamond layers a total of 48 plates were prepared as etalon plateswith lateral dimensions of typically 4.0×4.0 mm and thicknesses ofapproximately 1.25 mm.

The parallelism of 9 etalon plates were measured using a Zygo GPI phaseshifting 633 nm laser Fizeau-type interferometer, using the angleapplication included in the software of the Zygo-GPI interferometer.

Parallelism is calculated from the angle of the least-squares fittedplane to the wavefront of the beam transmitted through the etalon withrespect to the plane wavefront of the unperturbed beam. This defines awedge angle between the front and backside of the etalon

The average wedge angle α varied between 2.3 and 13.8 arcsec with anaverage value of:α_(av)=9.2 arcsecand a standard deviation of:σ_(α)=3.5 arcsec

Example 7

8 sets of homoepitaxial CVD diamond layers were synthesised according tothe method described in Example 1. The CVD diamond layers had lateraldimensions of up to 6.5 mm and thicknesses of up to 3.2 mm. From theseCVD diamond layers a total of 48 plates were prepared as etalon plateswith lateral dimensions of typically 4.0×4.0 mm and thicknesses ofapproximately 1.25 mm.

Surface flatness over one surface of each of the 48 etalon plates wasmeasured using a Zygo GPI phase shifting 633 nm laser Fizeau-typeinterferometer, using the flatness application included in the softwareof the Zygo-GPI interferometer.

In this application the flatness is defined as the largest peak-valleydifference, after fitting a plane to the measurement data. FIG. 8 showsthe measured surface flatness values.

The average surface flatness F is:F=51.1 nm, or 0.16 fringesand the standard deviation isσ_(F)=18.2 nm, or 0.058 fringes

Example 8

8 sets of homoepitaxial CVD diamond layers were synthesised according tothe method described in Example 1. The CVD diamond layers had lateraldimensions of up to 6.5 mm and thicknesses of up to 3.2 mm. From theseCVD diamond layers a total of 48 plates were prepared as etalon plateswith lateral dimensions of typically 4.0×4.0 mm and thicknesses of1.2510±0.00025 mm.

Surface roughness for 15 of these plates was measured with a ZygoNewView 5000 scanning white light interferometer using a 20×magnification interferometric objective of the Mireau type and the ZygoMetroPro software package. Zoom was set at 1×. Camera resolution was640×460 pixels with 20 Hz refresh rate. Field of view was 0.36×0.27 mmand the lateral resolution was 0.56 micron. A software bandpass filterwas used with cutoff spatial frequencies of 12.5 and 400 lines/mm. Aleast squares fit to the surface profile was made in which the piston,tilt, power and astigmatism related to the overall surface position,angular orientation and form were removed. The remaining deviations fromthe reference surface thus defined were filtered with the bandpassfilter and the root-mean-square value of the deviations was calculated.The roughness thus determined was found to vary between 0.5 and 1.5 nmbetween plates with an average value of 0.92 nm and standard deviationof 0.11 nm. Individual plates showed a much smaller standard deviationof less than 0.05 nm when measuring at 5 different randomly chosenpositions over the full 4.0×4.0 mm surface area

Example 9

A set of five samples was measured, of which four were single crystalCVD samples and one was a IIa natural diamond sample. The details of thesamples are given in Table 1 below.

TABLE 1 Samples used for scatter measurements Sample Thickness,Dimensions, Process (based on that Number ID μm mm of Example 1) 1 SM11352 4.61 × 4.28 5 ppm N₂, 2 SM2 1471 5.74 × 5.56 5 ppm N₂, 210 × 10² Pa3 SM3 428 4.96 × 3.34 1 ppm N₂, 4 SM4 912 6.60 × 6.09 0 N₂ (<300 ppb) 5AM1 942 5.17 × 3.36 Natural IIa diamond.

The samples were all carefully prepared with an optical polish withtheir major faces as close to [001] as is possible, typically within1.5°.

Prior to measurement, all the samples were cleaned in a stronglyoxidising mixture of hot concentrated sulphuric acid and potassiumnitrate to remove any traces of surface contamination. After cleaninggreat care was taken to avoid re-contaminating the surfaces withanything that could cause extraneous surface scatter effects.

The total scattering power (TSP) at 1.064 μm was measured and calculatedaccording to the method described earlier, and the values are shown inTable 2.

TABLE 2 TSP values at 1.064 μm for the five samples under investigationTotal Scattered Power, % Angular Range Sample 2.5°-87.5° 3.5°-87.5°4.5°-87.5° 1 0.122741 0.071681 0.06083 2 0.198536 0.162015 0.101446 30.144404 0.129007 0.118031 4 0.651108 0.517491 0.251997 5 0.6729360.445124 0.114724

This data shows that material grown without N deliberately added has asubstantially higher scatter than material grown with some nitrogenadded. This is consistent with observations that the stress level (asrevealed by birefringence) is higher. In comparison there appears to berelatively little difference between the three samples grown withdifferent levels of nitrogen in the process and different processpressures although there are small variations. The high values ofscatter for both the CVD grown without nitrogen and the natural type IIastone shows the particular benefit of CVD diamond synthesised by themethod revealed here, and as natural type IIa diamond is known tocontain stress and dislocations the scatter is presumably by a similarmechanism.

Example 10

A homoepitaxial CVD diamond layer was synthesised according to themethod described in Example 1. It was then prepared as an optical plate,E10.1, with six polished {100} faces and with dimensions of 4.00 mm×3.65mm×1.31 mm.

Sets of Deltascan images, each covering 1 mm×0.75 mm, were recorded at awavelength of 589.6 nm for each of the three mutually perpendicularviewing directions normal to the faces of the sample. Each Deltascan sinδ image was analysed for the maximum values of sin δ in the mannerdescribed earlier, using 100% of the data obtained within the frame.

The maximum value of sin δ for the sin δ map recorded with the viewingdirection parallel to the 4.00 mm dimension of the plate was 0.1.Likewise, the maximum value of sin δ for the sin δ map recorded with theviewing direction parallel to the 3.65 mm dimension of the plate was0.1. A maximum value of Δn_([average]), the average value of thedifference between the refractive index for light polarised parallel tothe slow and fast axes, was then calculated for each of these twoviewing orientations and found to be approximately 3×10⁻⁶.

The values obtained with the viewing direction perpendicular to the twolargest dimensions and parallel to the 1.31 mm dimension are shown inthe sin δ map below. This viewing direction corresponds to the growthdirection of the CVD diamond layer, and thus is parallel to the dominantdirection of dislocations in the material.

FIG. 13 is the Deltascan map of sample E10.1, with the viewing directionparallel to the 1.31 mm dimension of the plate, showing the maximumvalue of |sin δ| in each frame of 1 mm×0.75 mm.

The corresponding maximum values of Δn_([average]) for each frame inthis viewing direction can be calculated based on the corresponding sinδ value and the sample thickness, with the values given below:

Sin δ 0.10 0.20 0.30 Δn 7.2 × 10⁻⁶ 1.4 × 10⁻⁵ 2.2 × 10⁻⁵

In some of the more demanding optical applications, the presence ofrandomly scattered points, or even a single point, of higher stress maybe limiting. This data, using every data pixel measured across thesample, shows that material grown using the method of the invention canachieve extremely low levels of strain related birefringence bothgenerally across the sample and also locally.

Example 11

Quantitative luminescence measurements were made on a range of as grownand annealed single crystal CVD diamond samples grown according to themethod in example 1. In each case the measurements were made after theremoval of the {100} synthetic Ib substrate on which the sample wasoriginally grown. The growth conditions favoured the formation ofpredominantly <100> growth sector diamond material with uniformluminescence properties as judged by luminescence imaging. Any smalladditional growth sectors at the edge of the sample with differentluminescence properties were removed before the measurements were made.

The luminescence measurements were made at 77K using the methoddescribed earlier, and were normalised relative to the 1332 diamondRaman line, also as described earlier. The results obtained are shown inTable 3 below:

TABLE 3 Samples used for luminescence measurements Raman normalisedRaman normalised Sam- intensity of 575 nm intensity of 637 nm ple PLline PL line Annealing treatment 1 126.7 56.57 None (As-grown) 2 101.350.8 None (As-grown) 3 141.6 67.8 None (As-grown) 4 1.09 1.26 24 hoursat 1800° C., 75 kBar 5 1.99 0.76 4 hours at 1950° C., 75 kBar 6 0.170.22 4 hours at 2400° C., 80 kBar 7 0.16 0.63 24 hours at 2100° C., 80kBar 8 1.09 0.68 24 hours at 2250° C., 80 kBar 9 0.39 0.70 4 hours at2400° C., 80 kBar

The absolute value of the upper level of 575 nm and 637 nm PL intensityis in part determined by the nitrogen concentration in the growthprocess, but can thus be advantageously reduced by annealing asdemonstrated here.

Example 12

8 sets of homoepitaxial CVD diamond plates were synthesised according tothe method described in Example 1. From these sets in total 48 plateswere prepared as etalon plates with lateral dimensions of typically3.5×3.5 mm and thicknesses approximately 1.255±0.005 mm.

The optical homogeneity of 6 etalon plates, originating from 4 differentsets, were measured using a Zygo GPI phase shifting 633 nm laserFizeau-type interferometer. The scanned area was typically 3.2×3.2 mm.

An effective optical homogeneity is defined in terms of the largestpeak-to-valley variation (PV) of the wavefront difference of the beamtransmitted through the etalon and the unperturbed beam, after removalof the variations due to long-scale shape. The Zygo GPI interferometersoftware option to remove the tilt from the wavefront difference removesthe non-parallelism of the front and back face of the sample, whereasthe power and astigmatism software options remove the cumulative effectof the curvatures of the surfaces. In this way an effective maximumvariation in the refractive index over the measured area can be definedby the relation

${\Delta\; n} = \frac{{PV}\;({fringe}) \times {\lambda/2}}{d}$

In Table 4 below the measured PV values are tabulated for the case thatonly the tilt is removed and for the case that all shape factors areremoved. The lower PV values in the second case as compared to the firstindicates that there are some scale shape effects due to non flatness ofthe plate surfaces still present, although it can not be excluded thatalso some of the (large scale) variation of the refractive index isfiltered out in this way.

The effective optical inhomogeneity Δn over the plates, determined inthis way, is less than approximately 8×10⁻⁵ and more than approximately4×10⁻⁵.

The variation between plates from different growth sets is about 1×10⁻⁵.

TABLE 4 Samples used for optical homogeneity measurements PV (fringe) Δn(×10⁻⁵) Tilt, Curvature, Tilt, Curvature, Tilt Astigmatism TiltAstigmatism Sample removed removed removed removed 0005-2 0.262 0.2216.6 5.6 0044-8 0.285 7.2 0044-10 0.344 0.141 8.7 3.6 0053-11 0.290 0.2317.3 5.8 0052-2 0.320 0.279 8.1 7.1 0052-12 0.293 0.167 7.4 4.2 average7.6 5.3 standard 0.7 1.4 deviation

Example 13

2 sets of homoepitaxial CVD diamond plates were synthesised according tothe method described in Example 1. From these sets 50 uncoated etalonswere prepared with lateral dimensions of typically 1.5×1.5 mm andthicknesses approximately 1.250 mm. For 6 etalons the insertion loss wasdetermined by measuring the maximum transmitted light intensity I_(p) ofthe transmission spectrum of each etalon at the beginning and at the endof the C-band, thus at approximately 192200 GHz and 197200 GHz, using apinhole with a diameter of 1.2 mm positioned centered on the etalonfront face.

In a separate measurement the transmitted light intensity I_(o) wasmeasured at the same frequencies, without the etalon, but with thepinhole in place.

The relative difference in transmitted light intensities is defined interms of the insertion loss and is calculated according to equation 5,and set out in Table 5.

The insertion loss of real etalon is reduced compared to that of anideal etalon, which has an insertion loss of 0 dB, because of acombination of several factors, the most important being the absorptionof incident light in the bulk, surface scatter due to imperfect finishof the surfaces (surface roughness) and non-parallelism of the outerfaces. It can be deduced from different measurements on the contrastratio that for these etalons the non-parallelism and surface scatter canlargely explain the measured insertion losses.

TABLE 5 Samples used for insertion loss measurements Insertion loss (dB)Etalon Number @ 197200 GHz @ 192200 GHz 1 −0.39 −0.38 2 −0.34 −0.34 3−0.23 −0.23 4 −0.29 −0.34 5 −0.29 −0.28 6 −0.25 −0.23

Example 14

A range of single crystal diamond samples were synthesised according tothe general method of Example 1, the variations on this method beinggiven in the Table 6 below. After synthesis these samples were preparedas optical plates by careful surface polishing, resulting in thedimensions given. For comparison, optical grade polycrystalline diamondwith an optical polish was also included in subsequent measurements.

TABLE 6 Samples used for absorption measurements Thickness Diameter N2conc. Sample (μm) (mm) (ppm, N₂) Comments E14.1 1352 4.12 5 330 × 10² PaE14.2 606 5.08 1 E14.3 590 5.06 1 E14.4 1395 5.14 2.5 3.5% CH₄ E14.5 6085.09 0 Optical grade poly- crystalline diamond

Measurement of absorption was made as reported earlier, and the resultsare shown in Table 7 below:

TABLE 7 Samples used for absorption coefficient measurements AbsorptionCoeff. (cm−1) Sample At 10.6 μm At 1.064 μm E14.1 — 0.0483 E14.2 0.02620.0071 E14.3 0.0264 0.0077 E14.4 — 0.0464 E14.5 0.0362 0.119

Example 15

3 sets of homoepitaxial CVD diamond layers were synthesised according tothe method described in Example 1. The CVD diamond layers had lateraldimensions of up to 6.5 mm and thicknesses of up to 3.2 mm. From theseCVD diamond layers a total of 11 etalon plates were prepared withthicknesses varying between 1.250 mm and 1.258 mm. The thickness of eachplate was measured at several positions with a micrometer to an accuracyof better than ±0.5 μm. Over each plate no thickness variations werefound within this accuracy. The lateral dimensions of the plates areshown in Table 8.

TABLE 8 Samples used for FSR measurements shown in FIG. 9 ID numberLength (mm) Width (mm) 1 4.75 4.81 2 4.64 4.63 3 4.73 4.78 4 5.2 5.24 54.72 4.74 6 4.76 4.79 7 4.7 4.7 8 5.9 5.9 9 5.9 5.9 10 5.8 5.8 11 6.346.4

The FSR over the C-band (197200-192200 GHz) was determined on severalpositions (4 to 9 positions with average of 6.7) on each of the etalonplates by measuring the frequencies of the peak position of the etalontransmission peak nearest to the start and end position of the C-bandfor perpendicular incident light. The distances from each measurementposition to the next nearest measurement position was at least 1.6 mm. A1 mm diameter beam was used, defined by using a metal plate with anarray of pinholes fixed to the etalon plate. By moving the plate in thebeam by means of micrometers each of the positions could be illuminatedseparately. The effective FSR of each etalon is calculated by dividingthe frequency interval between the two extreme peaks by the number ofperiods in the transmission spectrum (For this etalon thickness andfrequency band typically 100 periods).

The average FSR over all positions on one plate was calculated for eachplate, as well as the standard deviation σ_(FSR) and the maximum errorΔFSR_(max).

This maximum error is defined as

${\Delta\;{FSR}_{\max}} = \frac{{\max\limits_{i}\left\lbrack {FSR}_{i} \right\rbrack} - {\min\limits_{i}\left\lbrack {FSR}_{i} \right\rbrack}}{2}$where the index i stands for the different positions on a plate.

The FSR of these plates was approximately 1.66 cm⁻¹. FIG. 9 shows thestandard deviation and the maximum error of the FSR for the differentetalon plates. The average standard deviation is σ_(FSR,av)=1.37×10⁻⁴cm⁻¹. The largest maximum error that was found is 3.5×10⁻⁴ cm⁻¹, whereasthe smallest error is as low as 6.7×10⁻⁵ cm⁻¹.

The invention claimed is:
 1. A CVD single crystal diamond material comprising: a low optical birefringence, indicative of low strain, such that when a sample of the material is prepared as an optical plate having a thickness of at least 0.5 mm and measured at room temperature, nominally 20° C., over an area of at least 1.3 mm×1.3 mm, |sin δ|, the modulus of the sine of the phase shift, for at least 98% of the measured area of the sample remains in first order, such that δ does not exceed π/2, and |sin δ| does not exceed 0.9.
 2. A CVD single crystal diamond material according to claim 1, wherein |sin δ| does not exceed 0.9 over 100% of the measured area of the sample.
 3. A CVD single crystal diamond material according to claim 1, wherein |sin δ| does not exceed 0.6 over at least 98% of the measured area of the sample.
 4. A CVD single crystal diamond material according to claim 1, wherein |sin δ| does not exceed 0.6 over at least 100% of the measured area of the sample.
 5. A CVD single crystal diamond material according to claim 1, wherein the diamond material has a single substitutional nitrogen concentration of more than 3×10¹⁵ atoms/cm³ and less than 5×10¹⁷ atoms/cm³ as measured by electron paramagnetic resonance (EPR).
 6. A CVD single crystal diamond material according to claim 5, wherein the single substitutional nitrogen concentration is less than 2×10¹⁷ atoms/cm³ as measured by electron paramagnetic resonance (EPR).
 7. A CVD single crystal diamond material according to claim 5, wherein the single substitutional nitrogen concentration is more than 1×10¹⁶ atoms/cm³ as measured by electron paramagnetic resonance (EPR).
 8. A CVD single crystal diamond material according to claim 1, comprising a low and uniform optical absorption such that a sample of a specified thickness of at least 0.5 mm has an optical absorption coefficient at a wavelength of 1.06 μm of less than 0.09 cm⁻¹.
 9. A CVD single crystal diamond material comprising: a low optical birefringence, indicative of low strain, such that when a sample of the material is prepared as an optical plate of at least 0.5 mm thickness and measured at room temperature, nominally 20° C., over a specified area of at least 1.3 mm×1.3 mm, for 98% of the area analysed, the sample remains in first order, δ not exceeding π/2, and a maximum value of Δn_([average]), the average value of the difference between the refractive index for light polarised parallel to the slow and fast axes averaged over the sample thickness, does not exceed 1.5×10⁻⁴.
 10. A CVD single crystal diamond material according to claim 9, wherein Δn_([average]) does not exceed 1.5×10⁻⁴ over 100% of the measured area of the sample.
 11. A CVD single crystal diamond material according to claim 9, wherein Δn_([average]) does not exceed 5×10⁻⁵ over 98% of the measured area of the sample.
 12. A CVD single crystal diamond material according to claim 9, wherein Δn_([average]) does not exceed 5×10⁻⁵ over 100% of the measured area of the sample.
 13. A CVD single crystal diamond material according to claim 9, wherein the diamond material has a single substitutional nitrogen concentration of more than 3×10¹⁵ atoms/cm³ and less than 5×10¹⁷ atoms/cm³ as measured by electron paramagnetic resonance (EPR).
 14. A CVD single crystal diamond material according to claim 13, wherein the single substitutional nitrogen concentration is less than 2×10¹⁷ atoms/cm³ as measured by electron paramagnetic resonance (EPR).
 15. A CVD single crystal diamond material according to claim 13, wherein the single substitutional nitrogen concentration is more than 1×10¹⁶ atoms/cm³ as measured by electron paramagnetic resonance (EPR).
 16. A CVD single crystal diamond material according to claim 9, comprising a low and uniform optical absorption such that a sample of a specified thickness of at least 0.5 mm has an optical absorption coefficient at a wavelength of 1.06 μm of less than 0.09 cm⁻¹. 